Completed research includes definition of ultrastructure of the apical epithelium-basal lamina-mesenchyme complex and early patterns of chondrogenesis in developing human limbs during stages 15 through 18. In addition, primary cell cultures of human limb mesenchyme from stage 15 embryos (termed HLM15) are established and their ultrastructure and cycle characteristics analyzed. Proposed research for the coming year (1973-1974) includes: 1. Investigation of differential growth of mesenchyme in vivo using H3-thymidine-labeled mitoses techniques for determination of growth centers during hand plate formation. 2. Introduction of molecular species known to bind or act at the cell surface (e.g. wheat germ agglutinin, concanavalin A) into human limb mesenchyme (HLM15) cultures to determine potential alterations in cell cycles, surface areas, volume distributions, etc., as monitored by flow microfluorometry (FMF). 3. Resolution of fine differences in composition of basal laminae and extracellular matrix during limb morphogenesis by techniques of electron microscope autoradiography and immunocytochemistry.